Direct detection and characterization of terrestrial (Earth-like) planets in orbit around nearby stars remains a tantalizing proposition. Planets are expected to be found from a few tens to a few hundred milli-arcseconds in angular separation from nearby stars and of an order 10−10 times dimmer in visible light and likely embedded in an unknown sea of scattered light from dust surrounding the star, and seen through local dust, in our solar system.
A multitude of coronagraphic techniques for the space-based direct detection and characterization of exo-solar terrestrial planets by increasing planetary contrast relative to starlight, allowing for angular separation of the planet light from its star, and ultimately spectroscopy of the planet, have been considered. These approaches generally include a single telescope with an internal starlight suppression scheme. Typical coronagraphs have internal shaped focal plane masks, and/or shaped or apodized pupil plane occulting masks, or complex shaped optics which emulate apodization, or an internal nulling interferometer, which block and/or diffract starlight thereby increasing the planet's contrast with respect to its parent star. Each of these internal methods have differing yet extreme requirements on wavefront, amplitude and polarization, generally requiring some form amplitude, wavefront, and polarization sensing and/or control with stressing optical and stability requirements. These stressing requirements are due to incompletely suppressed diffracted/scattered starlight leaking through to the focal plane, subsequently reducing the contrast of the planet with respect to the suppressed starlight.
An alternative technique is to place a starshade, i.e., external occulter 201, at some distance in front of the telescope 202 (see FIG. 2) to suppress the starlight, prior to entering the telescope 202, thereby mitigating stressing requirements in the telescope 202 system and relaxing it to that of more conventional space telescope technology. The external occulter 201 (see FIG. 2), at distance z in front of the telescope 202 along the line of sight to the stellar system (star 203) under study, creates a deep shadow and the telescope 202 resides within it. The starlight is suppressed and the light from the planet 204, off-axis relative to the line of sight, passes the edge of the occulter 201 and enters the telescope 202 aperture without reduction in throughput and independent of wavelength.
In FIG. 2, the “geometric” inner working angle (IWA) is shown as the ½ angle subtended by the occulter 201 as seen from the telescope 102, i.e., φIWA=W/2z, where W is the diameter of the occulter 201, and z is the separation of the occultere 101 to telescope 202; “geometric” since this is only true to the limit of geometric optics, i.e., as the wavelength tends to zero. In practice the “diffractive” IWA, defined herein as that angle at which the contrast of the planet 204 to suppressed starlight exceeds unity, is slightly smaller than the geometric IWA due to diffraction, and only a weak function of wavelength over the range of interest. The depth of suppression and focal plane contrast vary with wavelength, occulter 201 width, separation and telescope 202 aperture diameter.
The external occulter 201 can be realized in space by flying two spacecraft: one including the telescope system 202, and the other including the external occulter 201. Each spacecraft includes a spacecraft bus, attitude control, fuel and communications, and when acquiring a new target the telescope 202 just re-points but the occulter 201 must “fly” to the new line of sight to the target star 203, or alternatively both the telescope 202 and occulter 201 must reposition themselves. This levies additional requirements on the system and reduces the science duty cycle—however it may be the only viable approach with existing technology for direct planetary detection. This approach contains large design margins since suppression of starlight to 10−10 at the telescope 202 aperture yields a focal plane contrast significantly higher at the IWA.
An external occulter 201 typically contains hard edges, e.g., a circular disk, which causes Fresnel diffraction effects that tend to fill in, or brighten the shadow, thus, leaking starlight through the telescope. Previous occulters were large (200-800 meter) petaled external occulters placed at separations of 105-106 km for potential use with the Hubble SpaceTelescope (HST) for planetary detection, but were considered infeasible for HST due to its orbital configuration. Other occulters included more reasonably sized external occulters (˜70 m) at separations of 50,000-100,000 km, but with apodized transmission, i.e., graded transmission which is blocking in the center and changes continuously to transmitting towards it edge to better mitigate diffraction effects.
Hybrid approaches are also possible whereby an external occulter 201 performs partial suppression but subsequently cascaded with an internal coronagraph within the telescope 202. This approach would require more stringent telescope tolerances—increasing the telescope's 202 cost, but may allow a smaller, closer-in occulter 201 with relaxed tolerances and lower fuel mass, while increasing the science time since less time is required to “fly” to the next target star.
Hybrid approaches included hard-edged (circular and square) occulters coupled to an internal apodizer to theoretically obtain suppression levels of 1010 . In another approach, a square occulter was used to conduct a ground demonstration to suppress Polaris. In another hybrid approach, a hyper-Gaussian apodization scheme has been used, but it was noted that a binary petaled occulter can approximate an apodized occulter and this has been shown to ˜10−7 with broadband light. Further, circularly symmetric graded apodizers can be well approximated by shaped binary occulting masks.
In another approach, a constrained linear optimization has been used to design an optimal one dimensional (1D) radial apodization function that suppresses broadband (0.4-1.2 microns) while simultaneously maintaining the intensity in the telescope aperture at 10−1, and this apodized “Vanderbei” occulter has been approximated using a binary petaled occulter. This Vanderbei form for the external occulter is currently the most effective design for an occulter which performs all the suppression external to the telescope, and has the flattest spectral response. Indeed optimal occulter shapes have been designed that can achieve smaller inner working angles than conventional coronagraphs and yet have high effective throughput allowing smaller aperture telescopes to achieve the same coronagraphic resolution and similar sensitivity as larger ones.
Thus a complex trade space exists between science, technical feasibility and cast with respect to the design of an externally occulted coronagraph, and the present invention was developed to model the occulter with errors and show parametric simulations with the end goal of ultimately exploiting this complex trade space.